glamor: fix for 8bit fonts and PointSize on GLES 2
This is a second part of GLES2 fixes, which fixed font problems on ES2, if you do not have GLES3.
This contains some controversal change, which leads to correct rendering, but have a side effect by allowing formats not readable on GLES2 to be used as glTexImage2D, if we do not want to actually read them by glReadPixels.
Fixes #1362 (closed)
Fixes #1411 (closed)
Edited by Konstantin